Re: Problems with FindRoot and recursive functions

*To*: mathgroup at smc.vnet.net*Subject*: [mg89672] Re: Problems with FindRoot and recursive functions*From*: dh <dh at metrohm.ch>*Date*: Tue, 17 Jun 2008 00:38:44 -0400 (EDT)*References*: <g35g0m$9bv$1@smc.vnet.net>

Hi Daniel, this is a very common mistake. For speed, Mathematica tries first to simpify your expression symbolically. Therefore, you should take care not to evaluate your function with a symbolic argument. This can be done e.g. by: x1[t_?NumericalQ]:=... hope this helps, Daniel Daniel Kuang Chen Liu wrote: > Hello, > > I have a recursive function of the form > >>> x1[t_] := If[t < 0, {t, 1}, 0.5 + x1[t - 1]] > > which returns a list of length 2, and the first element has a root at t=0.5 >>> In[3]:= x1[0.5] >>> Out[3]= {0., 1.5} > > I want to use FindRoot to determine t0 such that x1[t0][[1]] == 0, so I use > the following code > >>> FindRoot[x1[t][[1]] == 0, {t, 0.5}] > > to which Mathematica complains > > During evaluation of In[6]:= FindRoot::nlnum: The function value > {False} is not a list of numbers with dimensions {1} at {t} = {0.5}. > > It would much appreciated if someone could tell me what is wrong with the > code. > > Daniel Liu > > -- Daniel Huber Metrohm Ltd. Oberdorfstr. 68 CH-9100 Herisau Tel. +41 71 353 8585, Fax +41 71 353 8907 E-Mail:<mailto:dh at metrohm.com> Internet:<http://www.metrohm.com>